Organic compounds are brought into an excited state by absorbing light. By going through this excited state, various reactions (such as photochemical reactions) are caused in some cases, or luminescence is produced in some cases. Therefore, various applications of the organic compounds have been being made.
As one example of the photochemical reactions, a reaction (oxygen addition) of singlet oxygen with an unsaturated organic molecule is known (refer to Reference 1: Haruo INOUE, et al., Basic Chemistry Course PHOTOCHEMISTRY I (Maruzen Co., Ltd.), pp. 106-110, for example). Since the ground state of an oxygen molecule is a triplet state, oxygen in a singlet state (singlet oxygen) is not generated by a direct photoexcitation. However, in the presence of another triplet-excited molecule, singlet oxygen is generated to achieve an oxygen addition reaction. In this case, a compound that is capable of forming the triplet excited molecule is referred to as a photosensitizer.
As described above, in order to generate singlet oxygen, a photosensitizer that is capable of forming a triplet excited molecule by photoexcitation is necessary. However, since the ground state of an ordinary organic compound is a singlet state, photoexcitation to a triplet excited state is a forbidden transition, and a triplet excited molecule is unlikely to be generated. Therefore, as such a photosensitizer, a compound in which intersystem crossing from the singlet excited state to the triplet excited state easily occurs (or a compound in which the forbidden transition of photoexcitation directly to the triplet excited state is allowed) is required. In other words, such a compound can be used as a photosensitizer, and is useful.
Also, such a compound often discharges phosphorescence. The phosphorescence is luminescence generated by transition between different energies in multiplicity and, in the case of an ordinary organic compound, indicates luminescence generated in returning from the triplet excited state to the singlet ground state (in contrast, luminescence in returning from a singlet excited state to a singlet ground state is referred to as fluorescence). Application fields of a compound that is capable of discharging phosphorescence, that is, a compound that is capable of converting a triplet excited state into luminescence (hereinafter, referred to as a phosphorescent compound), include a light-emitting element using an organic compound as a luminescent substance.
This light-emitting element has a simple structure in which a light-emitting layer including an organic compound that is a luminescent substance is provided between electrodes. This light-emitting element is a device attracting attention as a next-generation flat panel display element in terms of characteristics such as being thin and light in weight, high speed response, and direct current low voltage driving. In addition, a display device using this light-emitting element is superior in contrast, image quality, and wide viewing angle.
The emission mechanism of a light-emitting element in which an organic compound is used as a luminescent substance is a carrier injection type. Namely, by applying a voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from electrodes are recombined to make the luminescent substance excited, and light is emitted when the excited state returns to a ground state. As the type of the excited state, as in the case of photoexcitation described above, a singlet excited state (S*) and a triplet excited state (T*) are possible. Further, the statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
As for a compound capable of converting a singlet excited state to luminescence (hereinafter, fluorescent compound), luminescence from a triplet excited state (phosphorescence) is not observed but luminescence from a singlet excited state (fluorescence) only is observed at a room temperature. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
On the other hand, when the phosphorescent compound described above is used, the internal quantum efficiency can be improved to 75 to 100% in theory. Namely, a luminous efficiency that is 3 to 4 times as much as that of the fluorescence compound can be achieved. For these reasons, in order to achieve a high efficient light-emitting element, a light-emitting element using a phosphorescent compound has been developed actively (for example, Reference 2: Zhang, Guo-Lin, et al., Gaodeng Xuexiao Huaxue Xuebao (2004), vol.25, No. 3, pp. 397-400). In particular, as the phosphorescent compound, an organometallic complex using iridium or the like as a center metal has been attracting attention, due to its high phosphorescence quantum yield.